Semiconductor structure having air gap between gate electrode and distal end portion of active area

Abstract

A semiconductor structure includes a semiconductor substrate having a trench isolation region formed therein. A conductive gate electrode is buried in the trench isolation region. An air gap is disposed between the conductive gate electrode and the semiconductor substrate.

What is claimed is: 
     
       1. A semiconductor structure, comprising:
 a semiconductor substrate having a major surface; 
 a trench isolation region in the semiconductor substrate; 
 an active area surrounded by the trench isolation region in the semiconductor substrate, wherein the active area has a longitudinal axis extending along a first direction; 
 a first gate electrode buried in the active area and adjacent to a distal end portion of the active area; 
 a second gate electrode buried in the trench isolation region and adjacent to the distal end portion of the active area; and 
 an air gap between the second gate electrode and the distal end portion of the active area, wherein the air gap overlaps with an entire thickness of the second gate electrode, wherein the first gate electrode is a memory gate electrode and the second gate electrode is a passing gate electrode, and wherein the air gap is adjacent to the second gate electrode and is not adjacent to the first gate electrode, and wherein the air gap overlaps with an entire thickness of the second gate electrode along a thickness direction that is perpendicular to a top surface of the semiconductor substrate, and wherein the air gap is discontinuous along an extending direction of the second gate electrode under a top view. 
 
     
     
       2. The semiconductor structure according to  claim 1  further comprising a cap dielectric layer on the second gate electrode buried in the trench isolation region, wherein the cap dielectric layer seals the air gap. 
     
     
       3. The semiconductor structure according to  claim 2 , wherein the cap dielectric layer is a silicon nitride layer and has a top surface that is flush with the major surface of the semiconductor substrate. 
     
     
       4. The semiconductor structure according to  claim 1  further comprising a silicon oxide layer disposed on a sidewall surface of the distal end portion. 
     
     
       5. The semiconductor structure according to  claim 1 , wherein a width of the air gap ranges between 2 nm and 5 nm. 
     
     
       6. The semiconductor structure according to  claim 1  further comprising a first gate dielectric layer between the first gate electrode and the semiconductor substrate. 
     
     
       7. The semiconductor structure according to  claim 1 , wherein the second gate electrode extends along a second direction that is not perpendicular to the first direction. 
     
     
       8. The semiconductor structure according to  claim 1 , wherein the air gap is disposed only between the second gate electrode and the distal end portion of the active area as viewed under top view. 
     
     
       9. The semiconductor structure according to  claim 1 , wherein the air gap does not expose a tungsten layer of the second gate electrode.